It is already known that particles having one or two preferential elongation directions can be oriented by dispersion in a medium formed by long molecules which are themselves capable of receiving a preferential orientation. Thus, linear polarisers in the form of plates have been obtained by applying a polymer film coating to fine filiform or lamelliform metallic particles or to elongated molecules of dichroic pigments, and subsequently drawing this film. The long molecules of the polymer, which are disposed parallel to the drawing direction, impose their orientation on the dispersed material which thus absorbs any luminous vibration parallel to its elongation direction.
Mesomorphic materials also commonly called liquid crystals, which are also formed by long molecules, exhibit these same orienting properties with respect to particles dispersed within them. These long molecules arrange themselves over short distances to form microregions which, in the absence of external forces, are generally disorderly oriented relative to one another. By adequate wall treatments or by adding traces of a surfactant to the material, it is possible through the surface tension forces developed to obtain, between two supporting plates, thin layers in which these microregions are aligned in a single direction, the molecules having a uniform orientation which is generally parallel or perpendicular to the walls.
One interesting feature of the constituent molecules of mesomorphic materials is their strong polarisability which enables them to be oriented under the action of a continuous or alternating electrical or magnetic field and to entrain in their movement the particles dispersed within the material. However, whereas in the smectic-phase materials, the orientation obtained persists when the field is no longer applied, the molecules of the materials in the nematic or cholesteric phase return to their initial orientation when the field is interrupted.
These properties of the nematic or cholesteric phases have been utilised for forming optical modulators or data presentation systems which utilise the variations in absorption of a thin layer containing metallic particles or a dichroic pigment in suspension and subjected to the action of an electrical or magnetic field. In modulators of this type which use a nematic material, the quiescent state is either in disorderly orientation or is oriented in a direction parallel to the wall. In this latter case, the layers is illuminated with light polarised linearly in a direction parallel to the direction of alignment. Under the action of the field, the molecules arrange themselves into a homeotropic (i.e. perpendicular to the walls), orientation, causing the layer or those parts thereof which are subjected to the field to pass from the absorbent state to the transparent state. In modulators which use particles or pigments dispersed in a cholesteric material, the layer absorbs the natural light. Under the action of the field, the molecules right themselves perpendicularly to the walls, the helical structure is replaced by a homeotropic structure and the layer becomes transparent.
It is also known that thin layers of mesomorphic materials in the smectic phase can be used for data presentation purposes. The walls are treated to promote a uniform orientation of the molecules. The layer, which is cooled slowly from the liquid isotropic phase to the smectic phase, adopts this uniform orientation and thus appears before recording as uniformly transparent. A light beam, generally infra-red, modulated in intensity by the information to be recorded and focussed at the level of the layer, scans the layer. Its maximum intensity is calculated in such a way that, at the point of impact, the power absorbed by the layer causes this latter to pass into the liquid isotropic phase. The sudden cooling which follows results in the formation at these points of disorderly oriented and therefore strongly scattering microregions, whereas the points subjected during scanning to the minimum intensity retain their initial state and remain transparent. The information thus recorded in the form of scattering dots on a transparent background can remain intact for several weeks. It is erased by restoring the uniform transparent state by fusion, followed by controlled cooling. The image obtained may be viewed directly or projected onto a screen by means of an auxiliary light source and a strioscopic system.
In an U.S. Pat. No. 4,040,047 entitled "Erasable thermo-optic storage display of a transmitted image", HARENG et al described a data presentation system which also uses a thermo-optical recording process in a thin layer of a material in the smectic phase. In this system, a light beam of constant intensity scans the layer for successively causing the temporary fusion of each dot. The signal corresponding to the information to be recorded is applied in synchronism with the scan between two electrodes surrounding the thin layer. The various dots recrystallise into a structure which is the less disorderly, the more intense the field thus applied during cooling, and the recording is made in the form of more or less diffusing dots on a transparent background. Projection is carried out by means of a strioscopic system. Erasure is obtained by applying a voltage pulse which is considerably higher than the maximum values of the recording voltage, the field thus created restoring the layer to a uniformly oriented and therefore uniformly transparent state.
In a copending patent application filed on Dec. 13, 1975 under the Ser. No. 643,866, now abandoned, and entitled "Thermo-optic smectic liquid crystal storage display", L. THYRANT described another data presentation system which also uses a thermo-optical recording in a thin layer in the smectic phase. A treatment of the walls promotes the orientation of the layer in a first direction (for example parallel), whilst electrodes surrounding the layer enable it to be subjected to an electrical field which imposes on it a second orientation (for example homeotropic) perpendicular to the first. An initial voltage pulse imposes this second orientation on the layer and thus renders it transparent. The luminous power applied by a global projection of the image or by a modulated scanning beam enables the layer which remains in the smectic phase to be locally heated to the vicinity of the smectic phase/nematic phase transition point. During the subsequent cooling step, the layer readopts the first orientation imposed by the walls at the heated points. The connecting zones between regions of parallel and perpendicular orientation, which are microscopically disordered, are diffusing. The process only requires a relatively weak luminous intensity because the dots to be recorded do not change phase, but can only be used for recording half-tone images.